In recent years there has been a rapid growth in the ownership and use of mobile cellular telephones. However, a limitation of cellular telephones remains the restricted geographical coverage provided by cellular networks. For example, remote, sparsely populated areas may either suffer from very poor quality coverage or no coverage at all. This has led to the implementation of satellite telephone networks such as INMARSAT.TM. where a mobile telephone communicates directly with an overhead satellite, the satellite relaying signals to and from some fixed position earth station. It is likely that demand for satellite telephone services will increase providing that mobile terminals can be made small enough to be attractive to users.
The demands placed upon the radio transmitting and receiving components of a mobile telephone are extremely high in the case of satellite telephone systems. This applies especially to the antenna. It is envisaged that the antenna of choice for satellite telephones will be the quadrifilar helix (QFH) antenna (K. Fujimoto and J. K. James, "Mobile Antenna Systems Handbook", Norwood, 1994, Artech House, pp. 455, 457). A QFH antenna 1 is illustrated in FIG. 1 and comprises four inter-wound resonant helical antenna elements 2a to 2d. The elements are arranged around a common axis A with starting points 3a to 3d respectively, offset from one another by 90 degrees and short circuited together (by short circuit connector 4) at the top of the antenna 1. In a receiving mode, signals received by the helical elements 2b to 2d are phase shifted, relative to the signal applied to the first helical element 2a, by 90, 180, and 270 degrees prior to combining the signals. Phase shifting may be achieved, for example, using baluns as described in U.S. Pat. No. 5,450,093. When the antenna is used in a transmitting mode, this process is reversed, with a signal to be transmitted being split into four identical components, which components are then phase shifted prior to application to respective helical elements 2a to 2d.
FIG. 2 shows in axial cross-section the spatial gain pattern of a typical QFH antenna, where the axis A coincides with the longitudinal axis of the QFH antenna 1 of FIG. 1. This pattern can be thought of either as the radiating strength of the antenna in the transmitting mode or the sensitivity of the antenna in the receiving mode. The gain pattern of FIG. 2 corresponds to right circularly polarised signals, given that the helical elements 2a to 2d are left handed helices. If the helical elements are right handed helices, then the gain pattern of FIG. 2 would apply to left circularly polarised signals.
It is apparent that the gain of the QFH antenna is concentrated in the upper axial direction (as viewed in FIG. 1) as a main frontal lobe 5 which is generally hemispherical in shape. Only a small backward lobe 6 is present. The spatial gain characteristic of the QFH antenna is ideal for satellite telephones which must communicate with satellites in or passing across a hemispherical (or dome-shaped) region above the earth.
One drawback of the QFH antenna is its relatively large size. A typical QFH antenna may be ten centimetres long and has a diameter of two centimetres, the same order as the dimensions of a typical satellite telephone. In practice, where the antenna projects from the top of the telephone, the antenna can double the total length of the phone. In order to improve the portability of satellite telephones having QFH antenna, it is therefore desirable to be able to fold away the antenna when the phone is not in use. A folding antenna of this type is disclosed in EP0694985 where the antenna is coupled to the phone by a rotatable joint.
A satellite telephone 7 having a foldable antenna 1 is illustrated in FIG. 3, where FIG. 3A shows the antenna 1 in its extended position and FIG. 3B shows the antenna 1 in its folded position. When extended, and as has been described above, the spatial gain pattern of the antenna is optimised for communicating with an overhead satellite. However, this is not the case when the antenna is folded away and where the frontal lobe 5 of the gain pattern is directed towards the ground (at least when the phone is in the upright position). Whilst it may not be necessary to transmit signals from the satellite telephone to a satellite with the antenna 1 in this position, it will generally be necessary for the telephone to receive paging signals from a satellite so that the telephone can be alerted to incoming calls. However, the gain afforded by the backward lobe of the QFH antenna is unlikely to be sufficient to allow for this purpose when the antenna is folded away, even if, as may be the case, paging signals are transmitted at a higher power level than other satellite originating signals.
It has been proposed to overcome this problem by providing a second paging antenna in addition to the main QFH antenna. This additional antenna would be smaller than the QFH antenna but would be arranged such that its gain is always optimised for satellite communication. Whilst the gain of the antenna would not necessarily be sufficient to allow transmissions from the telephone to a satellite, it would be sufficient to allow the telephone to receive paging signals. This solution is undesirable however because it both increases the cost and the size of the telephone.